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Ask Harlan: November 12, 2003

October 1, 2003
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ASK HARLAN
YOUR RF & MICROWAVE TECH Q&A RESOURCE

You may know Harlan Howe from his twelve years as publisher and editor of Microwave Journal ®, or from his 34 years as a Microwave design engineer and engineering manager, or from his service as an IEEE fellow and past president of MTT-S.

Now, although semi-retired, Harlan is available to answer your questions about RF and Microwave engineering. If he doesn't have the answer, he will find an industry expert who does.

Click here to submit your question now.


FROM: Athanasios Tsitouras, Skatatron

Dear Harlan,

I'm a student at the Patras University in Greece, department of EE engineering and it's the 1ST time I'm working on an AGC loop for a tranceiver 5ghz. I use for this 3 components: a variable gain LNA, a directional coupler and a power detector, all at 5ghz. The problem is that i can't find a detector diode (shottky diode) at 5Ghz, for my rectifier. This happens because all the companies which I applied to sell their products under the condition of buying at least 2000 pieces. But the cost is too high since i need only one diode. Can you suggest to me manufacters who can make a better offer to their customers or manufacters who give samples of their products? If the samples-idea fails, a 20-30 pieces deal would be the appropriate for me.

Thanks for your time and best regards,

Thanasis.

--------------

Dear Thanasis,

I'm sympathetic to your problem. In my view, companies that don't help students are very short-sighted. I have spoken to Mr. Mel Field at M/A-COM and he has agreed to send you some free samples. You should contact him directly to work out the details. He can be reached at: fieldm@tycoelectronics.com.

Good luck with your project.


FROM: Mark Smith, Airnet Communications

With multiple PLL synthesizers locked to the same reference, what causes the relative phase to vary short term and drift over long term? I know the obvious sources of phase noise and temperature gradients. The application is phased arrays and minimizing relative phase calibration between elements. Thanks

--------------

Dear Mark,

You have named the two major sources of phase change. Phase noise is largely a function of semiconductor noise, particularly 1/f noise and is the largest contributor to short term phase stability. Temperature gradients and circuit aging contribute to long term phase drift.


FROM: Matteo Biggi, Officine Pasquali

Dear Mr Harlan,

I'm a young engineer and I'm going to design a directional broadwall coupler. I have learned the theory to design a multihole coupler with a certain coupling in a quite large band.

But this theory concerned the use of only one row of holes and in the project that I make, this row is spaced a quarter of the dimension 'a' of the guide from the wall.

I know that a lot of broadwall couplers, especially those with a hard coupling (3, 6, 10 dB) are designed with two rows of holes symmetric in the respect of the centre line of the guide.

My question is this: if I obtain a directional coupler with a 20 dB coupling made with 1 row of holes spaced 'a/4' from the wall, what happens if I make another symmetric row of holes at 'a/4' from the other wall of the guide?

Is it possible to value it?

May I have a coupling 3 dB harder (17 dB)?

Thank You very much!

--------------

Dear Matteo,

A second row of holes will tighten the coupling but without doing the calculations, I'm not sure it will be exactly 3 dB.

The design equations for multi-hole broadwall and sidewall waveguide couplers are given in section 1 of: Microwave Engineer's Handbook , Vol 2, T. Saad, Artech House, 1971.


FROM: James Rautio, Sonnet Software, Inc.

Hi Harlan,

In EM theory, E stands for Electric field, D for Displacement, M for Magnetization. H and B are magnetic field and flux. Do you have any idea why the letters H and B were selected?

--------------

Hi Jim,

Good to hear from you. The first reference to H waves that I have seen is Vol 10 of the MIT Rad Lab Series, McGraw-Hill,1951.

They use it to describe TE and TM modes in waveguide and refer to H-modes and the H plane of the waveguide.

In his book, Microwave Transmission Design Data , McGraw-Hill 1948, Ted Moreno wrote about TE waves, "Waves of this type are also called H-waves, as it is only the magnetic field that has an axial component." I suspect that H was used to distinguish the magnetic field mode from the TEM modes in Coaxial structures. I have no idea why B was selected for Flux except for the possiblity of avoiding confusion with F for frequency. Perhaps some reader with more gray hair than I have can offer some historical insight.

Incidently, the entire MIT Rad Lab Series is available on CD-ROM from Artech House.


FROM: Lev Stessin, Elta Systems

My question concerns microwave packaging, more exactly Hydrogen poisoning of MMICs. The question is : How to prevent Hydrogen poisoning of MMICs in hermetic packages. Is H2-3000 getter the only one in the industry?

Thank you,

Lev Stessin

Senior engineer
Microwave division
Elta Systems
Israel

--------------

Dear Lev,

While sealing the package in a vacuum helps there is still some long term outgassing from the package materials that requires the use of a getter. I am not familiar with commercially available getters, however, I suspect that one of the semiconductor processing publications may be able to help you.


FROM: John Scanlon, Pro Plastics Inc.  

Dear Harlan,

We are a plastic fabricator and distributor. We currently have a customer that is manufacturing a component that is exposed to microwave radiation. He currently is using a type 6 nylon to fabricate this part. He has asked us to recommend a less expensive material and we are not sure what type of properties that are required to make a plastic "microwave safe."

Please advise,
Usher Lejtman
Pro Plastics Inc.

--------------

Dear John,

Any dielectric material with a low dissipation factor will be OK. Materials such as polystyrene, Teflon, Nylon, composites filled with glass or ceramic powder, etc. A simple test is to put a piece of the material in a microwave oven for a minute or two, if it doesn't get hot, it should be OK.


FROM: Mark Smith, Airnet Communications

Hello Harlan;

How are you? Mark Smith here working again on microwave waveguide stuff, it seems it never ends. Well I need some help on a WR-112 loop coupler, I can't find any design information yet and I know you are the one to talk too. Can you help?

Thanks

--------------

Dear Mark,

A waveguide loop coupler is made by inserting a small loop, with one end grounded, at the end of a coaxial line, into the waveguide through a hole in the H-plane of the guide. It is sometimes referred to as an inductive loop. It is not directional. The coupling is a function of the loop size, the depth of insertion, the location in the b dimension of the guide and the rotation angle of the loop. Although I have made loop couplers, I have never seen any specific design information. We did it empirically. If you have access to one the the finite element or EM field solver software programs, you could analyze it, but it will probably take longer than "cut and try".


FROM: Kah Boon Loo, Agilent Technologies

Hi Harlan,

My name is Loo. I had came across a microwave magazine's name (MSN) from which I tried to retrieve the articles, could you pls help.

Thanks in advance.

- B.O. Weinschel, " Coaxial Connectors: A Look to the Past and Future", MSN, Feb 1990, pp. 24-31

- W.Oldfield, "ECM/EW designs to 40GHz and Above Accomplished in Coax", MSN, June 1984

Appreciate your time and efforts.

--------------

Dear Loo,

MSN is the abbreviation for Microwave System News , which stopped publication in the early 90's. Unfortunately, we do not have old copies of MSN in our library. You may be able to find the information in a public or university technical library if you use the magazine's complete name.


FROM: Rainee Simons

Hello Harlan,

I am looking for two Ka-Band high power ferrite isolators. Power handling 100 Watts CW, center frequency 32 GHz, bandwidth about 1 GHz, Waveguide WR-28.

Rainee

--------------

Dear Rainee,

We do not recommend specific manufacturers. I suggest that you look at the Circulators/Isolaters section of our on-line buyers guide .


FROM: Raghavendra Kulkarni, Bharat Electronics

My paper "Design Options for SAW IF Filters Used in Wireless Applications " is not listed in the Jan. 1998 issue on your website, even though you sent an appreciation letter to me mentioning that the paper was well received.

--------------

Dear Raghavendra,

It is mentioned in the online editorial index, however, we do not have all papers in the archive going back that far due to changes in file formats used at that time.

(Ed. Note : Your contributed article from October 2000, "Asymptotic Behavior of Cosine Windows ," is available on the Web site.)


FROM: Hosain Menshady, IEI

Hello

I have almost completed my research on combline filter with stripline structure. I intend to pursue my research on combline filters with microstrip structure. I deeply appreciate your assistance and recommendations on this subject.

Thanks in advance for you cooperation. Looking forward to hear from you.

Best regards,

--------------

Dear Hosain,

I suggest that you refer to: Microstrip Lines and Slotlines , Second Edition, Gupta, Garg, Bahl and Bhartia, Artech House, 1996, ISBN# 0-89006-766-X. I can also recommend: Microstrip Filters for RF/Microwave Applications , Hong & Lancaster, Wiley, 2001, ISBN# 0-471-38877-7.


FROM: Sudarshan Nelatury, Penn State, Erie

Dear Dr. Harlan:

I want to compare various dispersion models of microstrips. Where can I get epsilon_eff versus frequency (2-18GHz) data for sapphire, allumina, barium tetratitanate substrates for different W/h ratios. What formula is most reliable?

I want to extend the work of H. A. Atwater, and AK Verma+RajKumar.

Ref: H. A. Atwater, Test for microstrip disperison formula , IEEE Trans MTT Mar 1988, pp. 619-621.

A.K.Verma and RajKumar, "A new Dispersion formula for microstrip line ," IEEE Trans MTT, Aug. 1998, pp. 1183-1187.

Thank you Sir

Sudarshan

--------------

Dear Sudarshan,

You need to start with accurate knowledge of the basic dielectric properties of the substrate material, which are best obtained from the manufacturers. Calculation of epsilon Eff and dispersion calculations can be done with any of the commercially available design software packages. For a theoretical discussion I suggest that you refer to: Microstrip Lines and Slotlines , Second Edition, Gupta, Garg, Bahl and Bhartia, Artech House, 1996, ISBN# 0-89006-766-X.


FROM: Madhura Vingkar, MAC ELECTRO

How does a micorwave oven work?

--------------

Dear Madhura,

A microwave oven uses high power, (300 to 600 Watts in a typical home oven) microwave energy in a closed cavity, into which the material to be heated is placed. The EM energy penetrates the material and excites the molecules of the material causing them to vibrate and rotate, which in turn generates heat. Since the metal walls do not absorb any energy, the oven itself remains cool.


FROM: Jawad Al Satari, jojo  

Why do we use microwaves in communication?

--------------

Dear Jawadal,

Microwave frequencies are used for communication because they travel in a straight line and have a much wider information bandwidth than lower frequecy radio waves.


FROM: Stan Friskney

Stan Friskney

I am doing a microwave link budget. Under Digital Parameters, the software has an input field for threshold-to-interference (T/I) ratio. The manufacturer's data for the radio I will be using provides two T/I ratios, one for co-channel and one for adjacent channel T/I. These numbers are vastly different (34 and -8 dB). Which of these two values for T/I should I use?

--------------

Stan,

This is a communication systems question that is completely outside of the purvue of Microwave Journal or my personal knowledge. I suggest that you get the radio manufacturer to explain their specs to you. Sorry!

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